function wpdocs_unregister_tags_for_posts() {
     unregister_taxonomy_for_object_type( 'post_tag', 'post' );
    }
    add_action( 'init', 'wpdocs_unregister_tags_for_posts' );

I am going to add above code in my site-specific plugin, You can add in child theme’s functions.php file.

Site Specific Plugin Tutorial: //justlearnwp.com/wordpress-functions-php-plugin/
WordPress Child Theme Tutorial: //justlearnwp.com/wordpress-child-theme/